Hydrochloric acid resistant ferrous alloy containing nickel copper and tungsten

ABSTRACT

A CORROSION RESISTANT IRON-NICKEL BASE ALLOY HAVING GOOD CORROSION RESISTANCE IN HYDROCHLORIC ACID, CONTAINING ABOUT 25% TO 50% NICKEL, 0.5% TO 11% COPPER, 0.1% TO 16% TUNGSTEN, AND WITH A MINIMUM AMOUNT, NO MORE THAN ABOUT 1%, OF STRONG SURFACE OXIDE-FORMING ELEMENTS SUCH AS CHROMIUM.

United States Patent 3,627,513 HYDROCHLORIC ACID RESISTANT FERROUS ALLOY CONTAINING NICKEL, COPPER AND TUNGS'I'EN Lawrence R. Scharfstein, Reading, Pa., assignor to Carpenter Technology Corporation, Reading, Pa. No Drawing. Continuation of application Ser. No. 517,432, Dec. 29, 1965. This application July 7, 1969, Ser. No. 839,685

Int. Cl. C22c 39/54 U.S. Cl. 75125 3 Claims ABSTRACT OF THE DISCLOSURE A corrosion resistant iron-nickel base alloy having good corrosion resistance in hydrochloric acid, containing about 25% to 50% nickel, 0.5% to 11% copper, 0.1% to 16% tungsten, and with a minimum amount, no more than about 1%, of strong surface oxide-forming elements such as chromium.

This application is a continuation of my application Ser. No. 517,432, filed Dec. 29, 1965, now abandoned.

This invention relates to an acid resistant alloy and more particularly to a steel alloy having enhanced resistance to corrosion in hydrochloric acid, as well as other corrosive media such as phosphoric acid and sulfuric acid, both at room temperature and at elevated temperatures.

A corrosion resistant alloy is generally identified as one which, in a given corrosive medium, will not lose metal or will not be penetrated at a rate greater than would be tolerable in use. Quite unexpectedly, I have discovered that copper and tungsten together, in the amounts stated hereinafter, in the absence of an appreciable amount of chromium, that is 1% maximum, provides a relatively inexpensive steel alloy having a highly desired degree of resistance to hydrochloric acid and other media. More specifically, within its broad range, my alloy is readily balanced to provide a rate of loss of metal no greater than about 25 mils per year when immersed in by volume hydrochloric acid at a temperature of about 50 C. for at least two 48-hour periods. My alloy is also readily balanced within its preferred range to provide a rate of loss of metal no greater than about 25 mils per year when immersed in 30% by volume hydrochloric acid at a temperature of about 50 C. for at least three 48-hour periods. The unique alloy of my invention having such enhanced resistance to hydrochloric acid and other media can also be readily worked and formed into parts having good mechanical properties.

Hitherto available alloys characterized by good resistance to corrosion in hydrochloric acid are nickel base alloys containing upwards of about 55% nickel with relatively large amounts of molybdenum, that is, about 12% to 30%. Such alloys have proved to be difiicult to work, particularly because of their extremely narrow hot working temperature range. Thus, because of the relatively high cost and short supply of such alloys, they have left much to be desired.

The foregoing, as well as additional objects and advantages of my invention, will be apparent from the following description thereof.

Depending upon the physical and mechanical properties desired, my composition in its broader aspects may contain, in the approximate amounts indicated in weight percent:

Carbon up to .1 Manganese up to 8 Chromium (maximum) 1 Nickel 25 to 50 Copper .5 to 11 Tungsten .1 to 16 Preferably, the remainder of my composition is essentially iron except for incidental impurities in keeping with good commercial metallurgical practice.

When this alloy is prepared using, as for example, an electric arc furnace, deoxidizers such as silicon, manganese, aluminum, zirconium, magnesium or the rare earths may be used with the result that a small amount from about .01% to 1% of one or more of these elements may be present as an incidental impurity. Such elements as phosphorus and sulfur are held to residual amounts of a few hundredths of a percent. Nitrogen normally may be present as an impurity, but may be added in amounts up to about .4% or more when desired. Small amounts of titanium, up to about 2%, or columbium, up to about 1% may be included for the beneficial efiect upon the weldability of the composition. In keeping with the usual commercial practice, columbium is accompanied by some tantalum and therefore the percent stated for columbium is to be understood as including tantalum in the usual proportion.

: Additional elements may be added to my composition which, in keeping with good metallurgical practice, do not impair the desired properties of my composition.

My composition has good hot workability but additions of misch metal or boron may be used to provide even better hot workability. An addition of misch metal in amounts sufiicient to result in the retention of from about .10% to 30% misch metal provides improved hot workability without objectionably affecting the corrosion resistance of the alloy. Instead of misch metal, I also may utilize boron in amounts up to about .01% to improve the hot workability of the composition. Preferably, the alloy contains from about .003% to .007% boron and best results are achieved with a boron content of from about .003% to .005%.

Manganese in an amount up to about 2% may be included in my alloy because of its beneficial effect upon hot workability in preventing hot shortness. Larger amounts of manganese up to about 8% may be used to provide an essentially austenitic alloy, when it is desired to use the lower amounts of nickel together with the larger amounts of such ferrite-forming elements as tungsten.

Molybdenum is preferably not added to my composition and is kept to no more than normal residual amounts. While up to about 1% molybdenum can be present, when molybdenum is present in greater amounts, the resistance of my composition to corrosion in hydrochloric acid is adversely affected.

The tungsten and copper work together in my composition to provide a unique degree of corrosion resistance with substantially less nickel than hitherto used in alloys intended for exposure to hydrochloric acid in service. While the mechanism by which this uniquely enhanced hydrochloric acid resistance is achieved is not fully understood, I now believe that the alloy forms a tungsten-rich surface oxide which, when the part is immersed in hydrochloric acid, is superficially attacked by the acid. As the surface oxide is attacked, tungsten, which has been dissolved in the acid, in the presence of copper replates on the surface as is evident from the fact that almost pure tungsten compounds are found on the etched surface and less tungsten is found dissolved in the acid than would be expected from calculations based upon weight loss measurements. Whatever the precise mechanism may be, it has been found that parts made of my alloy rapidly develop a tungsten enriched surface on exposure to hydrochloric acid which minimizes and retards further attack by the acid.

Without an effective amount of copper in the alloy, the tungsten alone is not effective in providing the improved acid resistance characteristic of my composition. It is also necessary that strong surface oxide-forming elements be held to a minimum. It is for this reason that chromium must be held to no more than 1% to attain the unique acid resistance of my alloy. Beneficial effects are attained with from. about 0.1% tungsten and about 0.5% copper, but below those amounts there is insufficient tungsten and copper to provide adequate resistance to attack by hydrochloric acid for practical purposes. Best results are achieved, that is maximum reduction in the rate at which metal is lost in hydrochloric acid, when the larger amounts stated of tungsten and copper are present. Thus, about 10% to 15% tungsten and 5% to copper are preferred. Above about 16% tungsten and 11% copper, increasing difficulty is encountered in working the alloy because of the formation of undesirable tungsten and/or copper precipitates.

At least about 25% nickel is required in my alloy to ensure a substantially fully austenitic microstructure. Nickel, in amounts above about 50% does not have any further beneficial effect, particularly on the corrosion re sistance of the alloy in hydrochloric acid to warrant the added cost and the increasing difficulties in working the alloy resulting from the presence of such large amounts of nickel. I preferably utilize about 30% to 40% nickel for best results.

Because tungsten is a ferrite former, 1 preferably use the larger amounts of nickel with the larger amounts of tungsten to ensure a substantially fully austenitic microstructure. The presence of an undesirable amount of a second phase such as ferrite or martensite in the composition can be readily detected by optical microscopy and controlled within the limits stated. Experiments indicate that up to about 5% of the microstructure may be of a phase other than austenite without unduly atfecting the corrosion resistance of the alloy.

Thus, to provide a composition having outstanding resistance to corrosion in hydrochloric acid at relatively low cost and which can be readily cast and hot and/or cold worked to form parts, my composition, in weight percent, preferably consists essentially of about:

Percent Carbon (maximum) .05 Manganese (maximum) 2.0 Silicon (maximum) .5 Chromium (maximum) 1 Nickel 30 to 40 Copper 5 to 10 Tungsten 10 to 15 Molybdenum (maximum) 1 the remainder being essentially iron, except for incidental impurities.

Observing the foregoing criteria, my alloy is readily balanced within the stated broad range to provide both a substantially fully austenitic microstructure as is well known to those skilled in the art, and to attain a rate of loss of metal no greater than about mils per year when mmersed in 10% by volume hydrochloric acid at a temperature of about 50 C. for at least two 48-hour periods. In addition to maintaining its austenitic balance, it is also necessary to include copper and tungsten in the proper proportions each within its own range of about 0.5% to 11% copper and about 0.1% to 16% tungsten, and as the level of nickel is raised within its range of about 25% to 50%, the minimum amount of tungsten required is reduced. Copper, though not as powerful, is also an austenite-forming element like nickel and works with the nickel present to preserve the austenitic balance of the alloy when the larger amounts of tungsten are used at the lower nickel levels.

My alloy is readily prepared and worked in accordance with good standard commercial practice. No special heat treatment is required, although I preferably solution-treat or anneal the alloy at about 1,800 P. to 2,050 F. followed by an air or water quench. The following examples of my composition were melted and cast in the usual manner into ingots from which bars were forged. Specimens 1 /2 ins. x /z in. x A; in. were formed, annealed and surface-ground to provide a predetermined surface area; Alundum, 46 grit being used in the grinding. Heat treatment of the specimens was carried out as indicated for each example. Each of the specimens was carefully Weighed to within 0.0001 gram before and after exposure to the test environment and the corrosion rate in mils per year (m.p.y) was calculated. The corrosion rates reported hereinbelow are the averages obtained from at least two specimens of each of the respective examples.

EXAMPLE 1 As a specific example of my composition, a heat was melted and an ingot cast containing in percent by weight:

Percent Carbon .040 Manganese .82 Silicon .29 Phosphorus .001 Sulfur .002 Chromium .04 Nickel 25 .23 Molybdenum .01 Copper 2.05 Tungsten 4.90 Iron (1) 1 Remainder except for incidental impurities.

Test specimens were formed as previously stated hereinabove, annealing being carried out at 1,825 P. for onehalf hour followed by air cooling. Test specimens were immersed in test solutions of hydrochloric acid of three different concentrations at 50 C. After two 48-hour periods, the rate at which metal was lost was calculated and was found to be 25 m.p.y. in 10% by volume hydrochloric acid, 45 m.p.y. in 20% by volume hydrochloric acid and 82 m.p.y. in 30% by volume hydrochloric acid.

EXAMPLE 2 As another specific example of my alloy, a heat was melted and an ingot cast containing in percent by weight:

Percent Carbon i .035 Manganese .86 Silicon .36 Phosphorus .001 Sulfur .001 Chromium .06 Nickel 25.20 Molybdenum .01 Copper 4.04 Tungsten 4.93 Iron (1) 1 Remainder except for incidental impurities.

Test specimens were formed as described in connection with Example 1 and were tested in hydrochloric acid at 50 C. After two 48-hour periods, the rate at which metal was lost was calculated and found to be 21 m.p.y. in by volume hydrochloric acid, 35 m.p.y. in by volume hydrochloric acid and 60 m.p.y. in by volume hydrochloric acid.

EXAMPLE 3 As another specific example of my alloy, a heat was melted and an ingot cast in percent by weight:

Percent Carbon .03 1

Manganese .65 Silicon .3 5

Phosphorus .001 Sulfur .003

Chromium .01

Nickel 37.12

Molybdenum .01 Copper 1.93 Tungsten 2.0 Iron (1) 1 Remainder except for incidental impurities.

Test specimens were formed as previously described in connection with Example 1 (except that annealing was carried out at 1,950 F. for minutes followed by a water quench) and were tested in hydrochloric acid at 50 C. After three 48-hour periods, the rate at which metal was lost was calculated and found to be 60 m.p.y. in 30% by volume hydrochloric acid.

EXAMPLE 3A As another specific example of my alloy, a heat was melted and an ingot cast having essentially the same composition as that of Example 3 except that it contained 4.11% copper and 5.16% tungsten with a correspondingly smaller amount of iron.

Test specimens were formed as was described in connection with Example 3 and were tested in hydrochloric acid at 50 C. After three 48-hour periods, the rate at which metal was lost was calculated and found to be 43.5 m.p.y. in 30% by volume hydrochloric acid.

EXAMPLE 3B As another specific example of my alloy, a heat was melted and an ingot cast having essentially the same composition as that of Example 3 except that it contained 8.05% copper and 5.17% tungsten with a correspondingly smaller amount of iron.

Test specimens were formed as was described in connection with Example 3 and were tested in hydrochloric acid at 50 C. After three 48-hour periods, the rate at which metal was lost was calculated and found to be 39 m.p.y. in 30% by volume hydrochloric acid.

EXAMPLE 3C As another specific example of my alloy, a heat was melted and an ingot cast having essentially the same composition as that of Example 3, except that it contained 3.94% copper and 10.25% tungsten with a correspondingly smaller amount of iron.

Test specimens were formed as was described in connection with Example 3 (except that this time annealing was carried out at 2,000 E.) and were tested in hydrochloric acid at 50 C. After three 48-hour periods, the rate at which metal was lost was calculated and found to be 28 m.p.y. in 30% by volume hydrochloric acid.

EXAMPLE 4 As another specific example of my alloy, a heat was melted and an ingot cast containing in percent by weight:

Percent Carbon .03 Manganese .6-7 Silicon .35

6 Phosphorus .001 Sulfur .001 Chromium .03 Nickel 37.31 Molybdenum .01 Copper 8.01 Tungsten 10.3 Iron (1) 1 Remainder except for incidental impurities.

Test specimens were formed as described in connection with Example 1 but were annealed for 40 minutes at 2,000 F. followed by quenching in water. These specimens were tested in hydrochloric acid at 50 C. After three 48-hour periods, the rate at which metal was lost was calculated and found to be 22 m.p.y. in 30% by volume hydrochloric acid. In addition, similar specimens were tested in 85% by weight boiling phosphoric acid. After five 48-hour periods, the rate at which metal was lost was calculated and was found to be 23 m.p.y. Similar specimens were also tested in 20% by weight sulfuric acid at C. After five 4'8-hour periods, the rate at which metal was lost was calculated and was found to be 14 m.p.y. When similar specimens were tested in 10% by weight hydrochloric acid at 80 C., the rate of loss of metal was calculated after five 48-hour periods and was found to be 81 m.p.y.

EXAMPLE 5 As another specific example of my alloy, a heat was melted and cast containing in percent by weight:

Percent Carbon .013

Manganese .55 Silicon .24 Phosphorus .01 Sulfur .01 Chromium .10 Nickel 37.80 Molybdenum .10 Copper 10.16 Tungsten 15.53 Iron (1) 1 Remainder except for incidental impurities.

Test specimens were formed as described in connection with Example 1 but were annealed for 45 minutes at 2,000 F. followed by quenching in water. These specimens were tested in hydrochloric acid at 50 C. After five 48-hour periods, the rate at which metal was lost was calculated and found to be 17 m.p.y. in 30% by volume hydrochloric acid. In addition, similar specimens were tested in 10% by weight hydrochloric acid at 80 C. The rate of loss of metal was calculated after five 48- hour periods and was found to be 74 m.p.y.

The following five examples demonstrate the criticality of copper and tungsten in my composition and their synergistic effect on hydrochloric acid resistance when both are present:

EXAMPLES A B C D E *Remaindcr iron except for incidental impurities.

Small experimental ingots of each of the compositions of Examples A, B, C, D and B were prepared. Except for Example E, no difiiculty was encountered in forging the ingots to in. square bars. In the case of Example E, extreme difiiculty was encountered, and numerous cracks were seen in the finished forging. Test specimens were Examples HCl HCl 30% HC] The test data from Examples B and C show the eflect of each of the elements copper and tungsten alone on a base analysis represented by Example A. Example D, within the broad range of my composition, shows the effect of about 0.5% copper and 0.1% tungsten. It is to be noted that in the case of the composition of Example D which contains minimum levels of tungsten and copper, a larger amount of nickel, closer to 50%, is to be used in order to obtain a corrosion resistance of no greater rate of metal loss than 25 m.p.y.

Test specimens formed from Example E were immersed in by volume hydrochloric acid at 50 C. After five 48-hour periods, the rate at which metal was lost was calculated and found to be 25 m.p.y. Example E demonstrates that when excessive amounts of copper and tungsten are present, the forgeability of my composition is greatly impaired, and the hydrochloric acid corrosion resistance is also affected.

Hot and/or cold rolling has a beneficial effect on corrosion resistance probably through grain refinement. However, the test specimens used in carrying out the corrosion resistance tests of each of the foregoing examples were neither hot nor cold rolled, and therefore those results do not reflect the betterment of corrosion resistance resulting from the finer grain structure imparted by such working.

Because of its outstanding resistance to corrosion in hydrochloric acid, both at room and elevated temperatures, together with the relatively small amounts of nickel used, my alloy is especially well suited for use in fabricating parts intended for service involving exposure to hydrochloric acid. For example, containers may be readily fabricated for use as pickling vats in which articles are placed to be pickled in dilute hydrochloric acid.

Larger amounts of carbon than stated hereinabove adversely affect the hot workability of my alloy. However, when parts are to be formed as castings rather than wrought, then more carbon, up to about 1.5% may be present in my alloy.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.

What is claimed is:

1. A corrosion resistant iron-nickel base alloy having good corrosion resistance in hydrochloric acid at room temperature and at elevated temperatures consisting essentially of, in percent by weight, carbon up to about 1.5 manganese up to about 8%, chromium no more than about 1%, nickel about 25% to 50%, copper about 0.5% to 11%, tungsten about 0.1% to 16%, up to about 1% of any of the elements of the group consisting of silicon, aluminum, zirconium, magnesium and the rare earths, nitrogen up to about .4%, titanium up to about 2%, columbium up to about 1%, molybdenum up to about 1%, misch metal up to about 0.3%, boron up to about 0.01%, the remainder being substantially iron, and said alloy being so balanced that it is substantially fully austenitic and when immersed in 10% by volume hydrochloric acid at a temperature of about 50 C. for two 48-hour periods, it loses metal at a rate no greater than about 25 mils per year.

2. A corrosion resistant iron-nickel base alloy having good corrosion resistance in hydrochloric acid at room temperature and at elevated temperatures consisting essentially of, in percent by weight, carbon up to about 0.1%, manganese up to about 8%, chromium no more than about 1%, nickel about 25% to 50%, copper about 0.5% to 11%, tungsten about 0.1% to 16%, up to about 1% of any of the elements of the goup consisting of silicon, aluminum, zirconium, magnesium and the rare earths, nitrogen up to about .4%, titanium up to about 2%, colurnbium up to about 1%, molybdenum up to about 1%, misch metal up to about 0.3%, boron up to about 0.01%, the remainder being substantially iron, and said alloy being so balanced that it is substantially fully austenitic and when immersed in 10% by volume hydrochloric acid at a temperature of about 50 C. for two 48-hour periods, it loses metal at a rate no greater than about 25 mils per year.

3. A corrosion resistant iron-nickel base alloy having good corrosion resistance in hydrochloric acid at room temperature and at elevated temperatures, consisting in percent by weight essentially of nickel about 30% to 40%, copper about 5% to 10%, tungsten about 10% to 15%, no more than about 0.05% carbon, no more than about 2.0% manganese, no more than about 0.5% silicon, no more than about 1% chromium, no more than about 1% molybdenum, boron up to about 0.007%, the remainder consisting essentially of iron except for incidental impurities, and said alloy being so balanced that it is substantially fully austenitic and when immersed in 30% by volume hydrochloric acid at a temperature of about 50 C. for three 48-hour periods, it loses metal at a rate no greater than about 25 mils per year.

References Cited UNITED STATES PATENTS 3,184,577 5/1965 Witherell 128 W 3,318,423 5/1967 Dunki 75125 3,318,690 5/1967 Floreen 75125 HYLAND BIZOT, Primary Examiner 

